With PR #2640, a timer is used to trigger updating progress pane every 100 ms to improve progress record rendering performance (commit: 760a9c2). Before the change, _progPane.Show and _pendingProgress.Update used to be called on the same thread, and now they will be called on different threads. This causes race conditions -- for all type members that are used by both methods, they might be changed at the same time and the states may be corrupted.

The screenshot below is from a recent CI build on master branch. The exception was raised from RenderHelper, and that was because it was actually enumerating over PendingProgress._topLevelNodes and at the same time new node was being added to/removed from _topLevelNodes by Update.

/cc @iSazonov @lzybkr